Warm Audio WA-67 - Teardown

[moamps]

Do you think it would be necessary? I don't know. I guess only experimenting would answer the question.

I’m not sure if that’s necessary or not either. If the power supply was designed by a serious engineer (and in this case it probably isn't, but this is just a copy of the Stellar/Apex variant), then when changing these resistors, the values of the corresponding capacitors should also be changed. The PSUD would probably quickly show whether this is necessary or not.

The change in B+ voltage should also be preceded by a check of the polarization voltage of the capsule. Only if that voltage is too low, it would make sense to raise that voltage to 210V.

In his two extensive posts, Gus gave basic guidelines on how to calculate something and that the output capacitor rated voltage should be considered.

And to agree with Gus in this thought:
"I think it is better to not always just give the answer but "point" them in a direction to think about so maybe they will learn something."

 

[Tim Campbell]

Well I'll add that the guys at Warm Audio read this forum all the time. The reason you see the quality components in this mic that you do is because they literally specify parts that are recommended by forums like this. You do their R&D work for them. They are not the only company guilty of this

The funny thing is they will pay extra for a transformer if the internet says thats best but when the same forums don't seem so emphatic over a capsule they will set a limit of 100 USD for a capsule and buy what is 2nd or 3rd best as long as it is cheap or ask designers to give them something they can build in china.

 

[abbey road d enfer]

And to agree with Gus in this thought:
"I think it is better to not always just give the answer but "point" them in a direction to think about so maybe they will learn something."

Most of the times, that's how I behave, but sometimes when I see the discussion lacks focus, which is what I think it is (I may be wrong), I tend toexpose my own solution, hoping those who want to learn will ask me "how do you get there?". Guilty as charged.
Actually, I think operating the circuit at 190v instead of 210 is marginal.

 

[soapfoot]

Most of the times, that's how I behave, but sometimes when I see the discussion lacks focus, which is what I think it is (I may be wrong), I tend toexpose my own solution, hoping those who want to learn will ask me "how do you get there?". Guilty as charged.

Speaking as someone who has no background in electronics training, is not, has never been, and will likely never be a design engineer, I'm extremely grateful for this.

At least until recently, most of the time when I've come here it's been with a specific problem I need to sort out to finish that days' work. I'd suspect there are a fair few members who are grateful for the generous people who lend a hand in the clutch, as not everyone here has electronics as a career focus. There are probably a fair number of musicians and producers who come here hoping to find the information that will help getting by without going broke (and who give freely to others when their areas of expertise are called upon).

So in a forum such as this one, I don't think sharing expertise and leading by example is malpractice at all. Perhaps if you were teaching an upperclass course at a University it'd be more important to force students to go the long way 'round. But even then, I find that most who are curious are much more likely to maintain that curiosity when they feel invited-in and helped.

 

[Whoops]

these 68 ohm resistors look like 6.8k to me.

You guys are correct,
that's what happens when you trace a circuit later at night after a full day of working mixing music,
then you mess up the color bands of the resistors with the color tracks in ProTools.

PSU schematic corrected:

 

[Whoops]

Speaking as someone who has no background in electronics training, is not, has never been, and will likely never be a design engineer, I'm extremely grateful for this.

At least until recently, most of the time when I've come here it's been with a specific problem I need to sort out to finish that days' work. I'd suspect there are a fair few members who are grateful for the generous people who lend a hand in the clutch, as not everyone here has electronics as a career focus. There are probably a fair number of musicians and producers who come here hoping to find the information that will help getting by without going broke (and who give freely to others when their areas of expertise are called upon).

So in a forum such as this one, I don't think sharing expertise and leading by example is malpractice at all. Perhaps if you were teaching an upperclass course at a University it'd be more important to force students to go the long way 'round. But even then, I find that most who are curious are much more likely to maintain that curiosity when they feel invited-in and helped.

Excellent post Soapfood,
I agree 100% with you

Thanks

 

[Whoops]

Also does this supply have a -6VDC output or +6VDC output. Normally +6 with the use of a 317 and you would normally use a 337 for a negative supply.
If it is +6 how are they biasing the tube? An u67 schematic has a - heater voltage.

That's already been posted here Gus.
I posted the schematic of the mic some pages back with the B+ and Heater voltages

here it is:

 

[Whoops]

It looks like the 4.7uf cap is rated at 160V? Think about what happens at power up with a simple supply without a ramp up circuit. B+ up before the heater.

Measured the voltage at the capacitor at power up and got readings of values between 150V and 180v for a second, until the voltage value falls to 60V and then slowly increases to around 80V

After power down, the capacitor also sees a voltage ramp up from aprox 80v to over 160V before slowly decreasing

So Warm Audio seem to be testing the voltage rating limits of this capacitor.
I will replace it for an higher voltage rated cap regardless if I increase the PSU B+ voltage or not.

 

[Flatpicker]

That's already been posted here Gus.
I posted the schematic of the mic some pages back with the B+ and Heater voltages

here it is:

I’ll make a guess that Gus is pointing out since the cathode is connected to ground, the grid has to be biased below that, hence the heater’s negative 6V (I don’t guess the heater cares which way the current is flowing) is divided by R10 and R11 to form -1.5V~-1.8V, and biases the grid to that voltage via R8 and R9. I don’t think I’ve ever seen this configuration, but then again I don’t study these circuits much. Interesting, though.

Regarding the power supply, the original ’67 just has R1, R2, & R18 and C1, C2, & C3 (referenced R1, 3, 7 and C1, 3, 4 on the ‘67 schematic). Referencing the WA schematic you posted R1 would be 6.8K, R2 = 8.2K & R18 = 168K (best I can tell), and the rest omitted. I’m guessing Nu used the 168K to increase the load and lower the voltage instead of using the zeners, but who knows. Anyway, I’d try shorting out the 6.8Ks and maybe even reduce R1&2 like emrr said. (hint: LTSpice is your friend!) Just my 2 pennies, and great thread btw.

 

[emrr]

FWIW there’s a lot of old broadcast gear that goes over the filter cap voltage ratings momentarily, even with a tube rectifier. FWIW. Certainly not hard to change it either.

 

[Whoops]

FWIW there’s a lot of old broadcast gear that goes over the filter cap voltage ratings momentarily, even with a tube rectifier. FWIW. Certainly not hard to change it either.

The cap it's withstanding the power ups and power downs at the moment,
but I don't know about long term functionality...
If I increase the B+ to 210V as per U67 schematic, the voltage will be even higher so maybe it would be a good preventive measure to replace it with an higher rated cap.

I wanted to try Neumann cap values here also , Neumann uses 0,5uf or 1uf here, as I'm still not sure why did Warm Audio specifically increased the value of this capacitor to 4,7uf when they left the rest of the circuit unchanged.

 

[Whoops]

Anyway, I’d try shorting out the 6.8Ks and maybe even reduce R1&2 like emrr said. (hint: LTSpice is your friend!) Just my 2 pennies, and great thread btw.

Thanks,
I never used LTSpice, don't know how to use it.

I've been thinking in just winding more turns on the transformer secondary as that would not be hard to do and could be interesting to this thread to show how the Chinese Power transformer looks inside and how was it constructed.

 

[emrr]

Those PSU resistor values are so high, and so many sections, it’s simple to alter B+ there.

 

[Faderjockey]

so did you notice the Gotham being wired differently?
I have to c12/251 styles I built from Advanced Audio parts they have 2 Gotham cables..
I unboxed the WA67 and was going to test it out in studio with one of my cables so I didn't mess up the new one in case I sent WA67 back to Sweetwater.
it wouldn't work.. at first I was like what the hell new out of box.. and for some reason I just thought try the 7pin they shipped with it. Then it worked and I used. But I tested both mine again on wa67 and they don't work but work on my mics.. which are the Apex460 donner mics.
I'm keeping the mic.. but man it's funny how close it sounds to my 2 I built that are supposed to be more c12/251..

 

[Whoops]

Maybe in those cables the Ground is wired to different Pins.
In the WA67 Pin3 is ground, and maybe another pin is ground also, I don't remember from memory now, but I can check it out

 

[Gus]

FWIW there’s a lot of old broadcast gear that goes over the filter cap voltage ratings momentarily, even with a tube rectifier. FWIW. Certainly not hard to change it either.

I agree with this however the way MET caps can slowly fail(the self healing part) is what I was posting about.

 

[Whoops]

Those PSU resistor values are so high, and so many sections, it’s simple to alter B+ there.

For sure, it's simpler to change the resistor values. But I like to work on transformers and I'm also curious to see how was the Chinese transformer done.
One time I unwound a Chinese ouput transformer of a Golden Age Neve clone mic pre, I was amazed how badly it was done and how much did they skimp on the winding details and on all the parts (almost no varnish, almost no insulation, thin enamel layer, less turns)

Let's see, depends how much time I will have in the next weeks, I'm mixing 2 records at the moment and have already a mastering and another record to mix on the waiting line, I just go to the workshop in my free time or when I need to relax from music work.

 

[Whoops]

I agree with this however the way MET caps can slowly fail(the self healing part) is what I was posting about.

Is it something that is well know that MET caps can slowly fail in normal use?
Or are you referring that caps in genereal can slowly fail from exceeding their rating?

 

[abbey road d enfer]

https://passive-components.eu/capacitors-electrostatic-constructional-solutions/3/

 

[abbey road d enfer]

One time I unwound a Chinese ouput transformer of a Golden Age Neve clone mic pre, I was amazed how badly it was done and how much did they skimp on the winding details and on all the parts (almost no varnish, almost no insulation, thin enamel layer, less turns)

One of the cost advantages of dual bobbin xfmrs is that there is no need for insulation between primary and secondaries.
Of course there are a few drawbacks like lower efficiency, highe and radiated flux.